In an induction motor (hereinafter, referred to as "IM") driven at variable speeds, PWM inverters are often utilized for the drive. Speed feedback is often omitted in such systems due to cost and reliability advantages. When such inverters drive the IM without using a speed feedback loop, current hunting is liable to occur in a particular periodicity range under a light load and torque ripple is liable to result. The current hunting may in some cases disable the control of the inverter. The IM, especially when driven at slow speed, is influenced by the torque ripple. Such inverters produce too much vibration to be used for applications intolerant of vibration (for example, elevator drive). Consequently, to avoid excessive control capability in the low speed range, the effective variable speed range is usually provided at no greater than 20 to 1.
The cause of the ripple lies in a delay of ON/OFF operation of power switching elements (as shown in TU to TW and TX to TZ in FIG. 2) composing an output section of an inverter This delay requires the switching command for ON.fwdarw.OFF.fwdarw.ON operation of each switching element to have a further delay (because it is necessary to prevent positive and negative power sources from being short-circuited by means of the operation delay of a transistor). Such inverters therefore provide an artificially imposed dead time between switchings, which unfortunately produces a difference between a commanded output voltage and an actual output voltage and hence a waveform distortion. Further, since the operational delays of the power switching elements and control elements are subject to manufacturing variables, the difference produced between a theoretical control value and an actual output value causes a further waveform distortion as shown by a dotted line in FIG. 3.
The foregoing control unit thus cannot be used for applications intolerant of vibration such as an elevator drive.